Mammal of the masses (3)
I believe the answer is:
ass
'mammal' is the definition.
(ass is a kind of mammal)
'of the masses' is the wordplay.
'of' indicates the answer is hidden within the clue.
'ASS' can be found hidden inside 'the masses'.
(Other definitions for ass that I've seen before include "Long-eared animal" , "sap" , "One in Bible spoke" , "A donkey or an idiot" , "Stupid person or American bum" .)